1. Field of the Invention
In the field of paint resins and coatings the use of polyglycidyl ethers has proved successful owing to their outstanding technological properties. Particular areas of application have been opened up, especially also in the field of coatings, by the use of higher-molecular polyglycidyl ethers having softening points between 50 and 125.degree. C and epoxide equivalents between 440 and 6,000. The epoxide resins, which, in the coatings field, are available either in the form of powders or of solutions in organic solvents, can be crosslinked by means of carboxylic acid anhydrides or by means of dicyandiamide, when it is predominantly the epoxide groups of the polyglycidyl ether which react; it, however, also possible, above all in the case of epoxide equivalents exceeding 2,000, to crosslink them under hot conditions, to give cured coatings, by means of phenolic or melamine resins which are present in a mixture with the polyglycidyl ether to the extent of 10-60%, preferably 20-40%, in which case predominantly the OH groups present in the higher-molecular polyglycidyl ether react. Such systems of the type just mentioned are used above all in the field of container coatings.
Beside the indisputable advantages of powder coatings, a disadvantage which must be mentioned is, above all, their poor levelling which, even using the widely known levelling auxiliaries, cannot be improved to such an extent that the surface of the film is equal in quality to the surface which can be achieved with lacquer films made from systems containing solvents. Further disadvantages are the high investment costs of equipment for powder production and for powder coatings, and the poor flexibility in the choice of layer thicknesses; layer thicknesses below 70-75 .mu.m can hardly be achieved.
For these and other reasons, coating by means of systems containing solvents will remain indispensable for many areas of application. However, the disadvantages of the heavy pollution of the environment, caused by solvents which evaporate, become increasingly more serious and the construction, which is usually necessary, of expensive plants for final combustion and the effective loss of the solvent represent decisive points in a cost analysis. The use of systems containing solvents is completely ruled out in many cases for reasons of occupational health.
It was, therefore, the object of the present invention to provide aqueous coating agents for stoving, based on aqueous dispersions of epoxide resin mixtures and epoxide resin curing agents, and optionally pigments and other customary additives, the properties of these coating agents being adaptable to the most diverse practical purposes.
It was a further object of the present invention to provide aqueous coating agents for stoving, based on aqueous dispersions of epoxide resin mixtures, in the form of a solvent-free, aqueous, stable dispersion, optionally conjointly with cross-linking agents and other additives required for a particular application, for example agents for improving elasticity, in which coating agents the aliphatic OH groups and/or the epoxide groups should be available for crosslinking.
2. Prior Art
Although it has hitherto been possible to manufacture dispersions of various polymers in water, dispersions of this type have proved very unstable. Settling out has taken place within a short interval of time of a few hours to a few days. The polymer dispersions hitherto known also have poor film-forming properties, which are mainly due to the large particle sizes of the resin, which have hitherto been of the order of magnitude of 50 .mu.m and higher.
DT-OS No. 1,921,198 describes dispersions, and processes for their manufacture, the resin phase of which can also consist of epoxide resin. In that specification a dispersion which contains particles of an average diameter of 1-5 .mu.m is manufactured with the aid of a colloid mill, using quaternary ammonium salts as cationic extraneous emulsifiers. Apart from the fact that the process is expensive owing to the use of a colloid mill, the diameter of the particles is still very large.
Processes for the manufacture of stable aqueous epoxide resin emulsions which are completely free from solvent are also known, for example, from DT-OS No. 2,332,165. In that specification, however, a liquid epoxide resin which can be emulsified by means of non-ionic emulsifiers is used as the resin phase. However, owing to their low degree of condensation, liquid epoxide resins are not suitable for many applications; the use of nonionic emulsifiers is not successful with polyglycidyl ethers of a higher degree of condensation, especially with those which are solid at room temperature.
It is stated in U.S. Pat. No. 3,707,526 that water-soluble coating materials can be manufactured by reacting customary, water-insoluble epoxide compounds, such as, for example, diglycidyl ethers of bisphenol A, with dimethylolpropionic acid, optionally in the presence of other carboxlic acids.
The process leaves something to be desired, since the reactants must be heated for several hours in order to manufacture the product, which is then subsequently reacted with amines, such as, for example, alkanolamines, which render it water-soluble. The prolonged time of heating, which is required for the manufacture of the product mentioned above, is not only inappropriate for a commercial process, but is undesirable for other reasons, since it enables a spontaneous, exothermic polymerisation of the epoxide compound to take place, whereby an infusible, insoluble and crosslinked plastic is obtained, which cannot be used as a coating material. In addition, a large molar excess of acid relative to epoxide groups is employed (epoxide groups:acid = 1:2--3), which is undesirable.
U.S. Pat. No. 3,336,253 discloses resins which can be rendered soluble in water and which are reaction products from monoalkanolamines or dialkanolamines and various water-insoluble polymers, in particular epoxide polymers, which contain terminal groups which are reactive towards amines. The resulting products become water soluble by the subsequent neutralisation of the alkanolamine radical with an acid. The preferred reaction products contain one epoxide radical per molecule and are applied as coatings to various substrates. The coatings are subsequently crosslinked by self-polymerisation. One disadvantage of these coating materials is the presence of epoxide radicals, which, in the presence of traces of acid or basic materials, such as, for example, the alkanolamine radicals which are present at one end of each molecule, can undergo a self-polymerisation, crosslinked infusible materials being obtained. As a result of this the stability in storage of the coating compositions is greatly reduced. In the above-mentioned U.S. Pat. No. 3,336,253 it is stated that the stability in storage of the epoxide/alkanolamine reaction products can be increased by removing all unreacted epoxide groups, using various compounds, such as, for example, dialkanolamines. This procedure is undesirable since, in the course thereof, all the reactive sites for subsequent crosslinking, which are necessary for the manufacture of a durable, solventfree coating, are removed. In addition, these products can only be manufactured if very large quantities of dialkanolamines are employed (for example up to 28% of diethanolamine is used in the said U.S. Pat. No. 3,336,253). The resulting coatings are so unstable towards aqueous media that they are completely unserviceable for many purposes. The products manufactured according to DT-OS No. 2,415,100 also contain stoichiometric quantities of alkanolamines, relative to epoxide resin. Although the quantities of amine are reduced to approximately 5% in DT-OS No. 2,426,996, the dispersions still contain considerable quantities of solvent. The process mentioned is also not suitable for manufacturing solvent-free dispersions. Epoxide resin derivatives having the formula I or I', which are used in the present invention, are, however, not described in the literature discussed above.